1. Field of the Invention
The present invention relates to a system for displaying three dimensional images. More particularly, but not exclusively, the present invention relates to improvements in such systems in the areas of electron beam deflection transformations, beam registration techniques, interlacing techniques, data processing and filtering, signal processing and methods and apparatus for providing inbuilt security and/or a unique identification means.
2. Description of the related Art
Reference is made to PCT/NZ93/00083, which is herein incorporated by reference. PCT/NZ93/00083 broadly relates to a system consisting of a rotating phosphor coated screen and one or more electron guns writing images to the screen (the `cathode ray sphere`). As the screen sweeps out a display volume within an evacuated chamber, the one or more electron guns fire at the screen to excite the phosphor coating to produce illumination at the required regions in three dimensional space.
This specification will adopt the terminology of PCT/NZ93/00083 where applicable. Each point illuminated in the display volume is referred to as a "voxel". A voxel is a point in three dimensional space which forms part of a three dimensional image. A voxel is a three dimensional analog to a pixel in a two dimensional display system, for example: a computer monitor.
As discussed in PCT/NZ93/00083, it is known to calculate the required X and Y deflection angles of an electron beam by means of trigonometric expressions involving the radial and vertical locations of the voxel in the display volume, the screen angle at that point and the position of the electron gun relative to the display volume. However, this approach assumes that the gun is located precisely at the position and orientation assumed in the transformation equations. In practice, the gun will be slightly misaligned leading to errors in the position of the voxels if the ideal trigonometric expressions are employed. Attempting to take such misalignment in to account by trigonometric expressions leads to very complex and computationally expensive equations for the deflection coordinates. The level of precision in gun positioning required by this display makes reliance upon mechanical alignment undesirable. The abovementioned application also described deflection equations which assumed that the electron guns were on the equator and the positions known accurately. It would be desirable to be able to calculate deflection values for the guns above or below the equator.
To detect the presence of the electron beam impinging on the screen, and more particularly to detect the orientation of the screen at any particular time, NZ/93/00083 describes a technique whereby a conductor is affixed to the perimeter of the screen. As the electron beam passes over and strikes the conductor a current pulse is produced in the conductor. The current pulse is processed by a detection circuit. The technique described in PCT/NZ93/000083, of placing a wire around the screen periphery is problematic in that it is difficult to accurately and securely place such a conductor onto the screen in such a way that rapid rotational movement of the screen will not displace it and so that it does not cause any visual obstruction of the image. Also, as the electron beam passes over a thin conductor the current "spike" produced may be difficult to extract from a noisy background and therefore the accuracy of the screen position derived from the abovementioned measurement will be reduced.
It has also been found that charge buildup on the screen may result in distortion of the electron beam as a result of electrostatic repulsion. Accordingly, it would be advantageous to provide an exit path for the electrons from the beam which are incident at any point on the screen.
To avoid addressing the screen at acute angles, the changeover between guns addressing the screen could be made gradually. This is in contrast with PCT/NZ93/00083where the changeover is effected abruptly at a specific screen position.
The display unit of the present invention may incorporate information specific to that device and be controlled by proprietary software. It is therefore desirable that there exist some secure means of validating the use of such a peripheral and/or by which information relating to the characteristics of the screen may be communicated to the controlling computer. This may also be particularly appropriate where specialised control software is sold with a display system. In this situation, it is an advantage to be able to uniquely identify a display device in order to match it with a particular piece of software.
Many techniques have been used to date to prevent the unauthorised use of software or apparatus. These techniques include the use of protected software, dongles, etc. These approaches have had mixed success, and are not well suited to provide security for the cathode ray sphere. It is therefore desirable to incorporate some means of identification in the three dimensional display system.
To reduce image flicker, a further extension to PCT/NZ93/00083comprises implementing a technique analogous to interlacing employed on two dimensional raster scanned terminals.
It has been found that the contrast of an image produced in a cathode ray sphere is adversely affected by the presence of background or extraneous light entering the display volume from behind the volume (with respect to the observer). It is therefore desirable to reduce the amount of this background illumination and thus enhance the image contrast.
It is an object of the present invention to provide a three dimensional display system which produces true three dimensional images accurately in such a way as to overcome the above mentioned difficulties, or to at least provide the public with a useful choice.